System and method for secure wireless connection

ABSTRACT

A system and method for implementing a wireless communications link between two equipment envelopes through inductive coupling include two “U” shaped inductors. Wireless communications is achieved via mutual inductance between the two inductors. Circuitry is provided, which enables the transmitter to determine if a receiver is within communications range. When the two envelopes are aligned, a dual-gap toroidal core is formed, in which the gaps have opposite polarity. The opposing polarity of the two gaps minimizes the far field signature of the transmitter/receiver pair, thus enhancing communication security.

BACKGROUND

[0001] The present invention is generally related to short rangewireless communication systems, and more specifically related to shortrange wireless communication systems utilizing inductive coupling.

[0002] It is often desirable to conduct communications between two selfcontained portions of a system, such as a military man portable radioand associated equipment used to load information (e.g., configurationparameters) into the radio. Typically, many types of military equipment(e.g., man portable radios) require military hardening (e.g., enhancedvibration, shock, environmental, electrical specifications) andwatertight integrity (e.g., submersible equipment), thus warranting selfcontained portions.

[0003] Several disadvantages are associated with typical systems, whichrequire that communications between the pieces of equipment be conductedvia wired cables and electrical connectors, such as bayonet lockconnectors and threaded connectors, for example. One disadvantage isthat, in the field, carrying cables and/or connectors can place a burdenon an operator. The cables/connectors are heavy and bulky, thus tiringthe operator and slowing his reactions. Also, the size of thecables/connector can result in the operator becoming less covert.

[0004] Another disadvantage is that in the field, or during an operationin which little time is available, connecting and disconnecting theportions of the system can take too long, thus possibly jeopardizing themission.

[0005] Another disadvantage is that these types of cables/connectors aresubject to corrosion and interference with the operation of theconnector (e.g., dirt in the threads) due to weather conditions andoperational requirements, such as being submersed in water and/or mud,for example.

[0006] Many military situations require covert operations, includingsecure communications between portions of the system. Due to the covertnature of many military operations, the cables connecting the portionsof the system must be shielded to prevent unauthorized disclosure of theinformation being transferred. An associated disadvantage is thatshielded cables/connectors tend to be heavy, bulky, stiff, and difficultto quickly connect and disconnect.

[0007] Wireless systems have been explored. However, many wirelesssystems require that the structure (envelope) of the portions of thesystem be modified to facilitate communications. For example, a windowmay be put into a metallic envelope to support an infrared link. Aassociated disadvantage is that the envelope of the equipment iscompromised.

[0008] Many wireless systems do not adequately address the requirementfor secure communications. For example, optical wireless systems may besubject to unauthorized monitoring/access simply by being visuallyobserved. Furthermore, typical wireless systems utilizingelectromagnetic communications means transmit signals which are alsoeasily subjected to unauthorized access.

[0009] An improved secure communications connection is desired.

[0010] In one embodiment, a secure wireless communication systemcomprises a first portion and a second portion. The first portioncomprises a first inductor and the second portion comprises a secondinductor. The first inductor comprises a first end and a second end. Thesecond inductor comprises a third end and a fourth end. The firstportion and the second portion define a gap therebetween. Wirelesscommunication is achieved across the gap through mutual inductancebetween the first inductor and the second inductor by aligning the firstend with the third end, and the second end the said fourth end.

[0011] In another embodiment, a method for providing secure wirelesscommunications includes aligning the first portion of the communicationsystem having the first inductor with the second portion of thecommunication system having the second inductor. When aligned, the firstand second inductors form a dual-gap toroidal core. A wirelesscommunication signal is conveyed across the gaps between the first andsecond portions via the first and second inductors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the Figures:

[0013]FIG. 1 is a cross sectional view of wireless secure connectionillustrating a separation between the first and second inductors inaccordance with an embodiment of the present invention;

[0014]FIG. 2 is a combination schematic diagram and cross-sectional viewof a system comprising a wireless secure connection in accordance withan embodiment of the present invention;

[0015]FIG. 3 is a plot comparing attenuation as a function of range forvarious range/attenuation relationships, in accordance with anembodiment of the present invention;

[0016]FIG. 4 is a plot comparing attenuation as a function of range fordifferences of various range/attenuation relationships, in accordancewith an embodiment of the present invention; and

[0017]FIG. 5 is a flow diagram of a process for providing wirelesssecure communication in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

[0018] An embodiment of a secure wireless communication system inaccordance with the present invention comprises two separate portions,wherein each portion includes a “U” shaped inductor. When the twoseparate portions are aligned, the two inductors form a transformerhaving a dual-gap toroidal core. Wireless communication is conductedbetween the two portions via mutual inductance across the two gaps ofthe dual-gap toroidal core. This embodiment allows each portion to beself contained, which is particularly advantageous in militaryapplications requiring military hardening and/or a watertight integrity.Secure communications are facilitated by the low power level of thesignal conveyed via mutual inductance and by signal loss due to eddycurrents formed in the walls of each portion. Secure communications arealso facilitated because radiation of electromagnetic energy from eachgap of the dual-gap toroidal core has an opposing polarity, thusessentially canceling radiating electromagnetic energy in the far field.As described in more detail below, attenuation of radiatedelectromagnetic energy from the two gaps is proportional to 1/r³,wherein r is the distance from the gaps, thus providing more securecommunications than existing systems having attenuation proportional to1/r².

[0019] Referring now to FIG. 1, there is shown a cross-sectional view ofa wireless secure connection 100 comprising a first inductor 16 andsecond inductor 18 separated by a gap distance 27, illustratingwirelessly conveyed signal 25. The first inductor 16 comprises a firstend 20 and a second end 22. The second inductor 18 comprises a third end24 and a fourth end 26. Inductors 16, 18 are shown as “U” shaped. Thefirst end 20 and the third end 24 define a first gap 23 therebetween andthe second end 22 and the fourth end 26 define a second gap 29therebetween. However, each inductor 16, 18 may vary in shape and size,while maintaining a configuration having ends 20, 22, 24, and 26,respectively. For example, each inductor 16, 18 may be rectilinear,arcuate (“U” shaped being a subset of arcuate), or a combinationthereof. The signal 25 is indicative of information communicated viamutual inductance between the inductors 16, 18.

[0020] Also shown in FIG. 1 is first portion 12 comprising the firstinductor 16 and second portion 14 comprising the second inductor 18.Each portion, 12 and 14, is a separate self contained unit. Eachportion, 12 and 14, may comprise, for example, a transmitter, areceiver, or a combination thereof. In one embodiment, the portion 12 isa military radio, such as a man portable military radio, for example,and the portion 14 is a faceplate and/or headset configured to provideinformation to the military radio 12 such as configuration/operationalparameters (e.g., modulation type, transmit frequency, receivefrequency), cryptographic keys, or a combination thereof. To aid inensuring that the signal 25 is conveyed via mutual inductance betweenthe first and second inductors 16, 18, the first and second portions 12,14, are aligned.

[0021] Aligning the first portion 12 with the second portion 14comprises aligning the first end 20 with the third end 24 and aligningthe second end 22 with the fourth end 26. The first portion 12 and thesecond portion 14 are aligned to facilitate the conveyance of signal 25between the respective ends, 20, 22, 24, 26, of inductors 16, 18. Thefirst portion 12 and the second portion 14 may be aligned by anyappropriate means. For example, one of the walls (wall 1 or wall 2) maycomprise a protrusion, or a plurality of protrusions. The other wall maycomprise a conformably shaped corresponding indentation, or plurality ofindentations, (e.g., pimple and dimple) configured to receive eachrespective protrusion, such that when the protrusion(s) are insertedinto the indentation(s), the first and second portions 12, 14, arealigned. Examples of other types of alignment means include slots,fasteners, visually aligning characteristics of the first and secondportions 12, 14, (e.g., aligning edges of portions 12, 14, to be flush),or a combination thereof.

[0022] Wall 1 and wall 2 represent sides of portion 14 and portion 12,respectively. Each wall may comprise any appropriate material allowingelectromagnetic energy to traverse therethrough. For example, each wall(1, 2) may comprise steel, iron, aluminum, plastic, ceramic, orcombination thereof (including alloys). In situations where securecommunications are desired, such as military scenarios whereinclassified data is being communicated, it is advantageous for the signal25 to be conveyed between the first portion 12 and the second portion 14with as little error as practicable. However, it is also advantageous ifthe wireless signal 25 is attenuated as the distance from the source ofthe electromagnetic energy (e.g., the gaps 23, 29) increases. Thus,allowing the signal 25 to wirelessly convey information across the gaps23, 29, but also preventing unauthorized access/monitoring of the signal25 from a monitoring point a distance away from the gaps 23, 29.

[0023] For example, the first portion 12 may comprise a man portablemilitary radio for communicating secure information with other radios.In the field, this radio may have to be reconfigured (e.g., changefrequencies, provide cryptographic keys, change modulation type).Accordingly, the second portion 14 may be configured as a faceplatewhich fits on the front panel of the radio 12. When the faceplate 14 isaligned with the front panel of the radio 12, reconfiguration parametersmay be wirelessly conveyed (e.g., via signal 25) to the radio 12 in asecure manner via mutual inductance. Furthermore, the configurationparameters may be predetermined, such that the radio operator need onlyactivate a switch to select the set of configuration parameters to beconveyed to the radio 12 from the faceplate 14. This exemplaryconfiguration of secure wireless connection 100 is particularlyadvantageous in situations where the radio operator is in a hostileenvironment and desires to keep his hands on his weapon.

[0024]FIG. 2 is a diagram of a communication system 200 comprising thesecure wireless connection 100 and amplification and detectioncircuitry, in accordance with an embodiment of the present invention.System 200 comprises input signal 44, input amplifier 38, exclusive orgate 30, comparator 40, sense resistance 32, a voltage divider circuitcomprising resistance 34 and resistance 36, termination resistance 28,output amplifier 42, and output signal 46. The herein description ofsystem 200 is in terms of the first portion 12 being a receiving portionand the second portion 14 being a transmitting portion, however, it isto be understood that the system 200 is not limited to thisconfiguration. In one embodiment, the first portion 12 is a receivingportion (receiver) of the communication system 200 and the secondportion 14 is a transmitting portion (transmitter) of the communicationsystem 200. In this embodiment, input signal 44 is provided to thetransmitting portion 14 and is conveyed, via wireless signal 25, to thereceiving portion 12. The input signal 44 is indicative of informationto be conveyed to the receiving portion 12, such as the configurationparameters described above, for example. The input amplifier 38amplifies the input signal 44 and provides amplified signal 45 to thesecond inductor 18 via sense resistance 32. The input amplifier 38 mayalso filter the input signal 44. The amplified input signal 45 isprovided to the second inductor 18 and transferred, via mutualinductance via the wireless signal 25 across the gaps 23, 29, to thefirst inductor 16. The conveyed signal 47 is provided to the outputamplifier 42. The output amplifier 42 amplifies, and optionally filters,the conveyed signal 47 to provide output signal 46. It is envisionedthat other embodiments of the system 200 may comprise various types ofcircuitry for performing signal processing, such as filtering, errordetection, error correction, noise reduction, gain control, or acombination thereof, for example.

[0025] Another embodiment of the system 200 comprises detectioncircuitry for determining if the first portion 12 and the second portion14 are within communication range of each other. Detection circuitry isadvantageous in scenarios including secure communications. For example,if a radio operator wants to transmit configuration parameters to aradio via the wireless communication system 200, if the receivingportion 12 is not within communication range of the transmitting portion12, secure information may be transmitted into the air, and possibleintercepted by an unauthorized user. Detection circuitry, fordetermining if the first portion 12 and the second portion 14 are incommunication range of each other, and for providing an indication ofsame, helps prevent this type of unauthorized access.

[0026] Detection circuitry, as depicted in FIG. 2, comprises the senseresistance 32, the voltage divider circuit comprising resistances 34 and36, comparator 40, exclusive or gate 30, and termination resistance 28.It is to be understood that this configuration of detection circuitry isexemplary and that other configurations of detection circuitry areenvisioned, such as via optical means, acoustic means, other electroniccircuits, or a combination thereof, for example, wherein the goal is todetermine if the first portion 12 and the second portion 14 are withincommunication range of each other. In operation, a detection signal isprovided to the second inductor 18. This detection signal may be asignal specifically designated for determining if the first and secondportions 12, 14, are within communication range of each other, may be acomponent of the information being conveyed from the first portion 14 tothe second portion 12 (e.g., signals 44, 45), may be a specificallydesigned detection interspersed with the information, or a combinationthereof. The detection signal is provided to the second inductor 18 viasense resistance 32. The voltage divider circuitry (resistances 34 and36) provides a predetermined portion of the detection signal to inputterminal 40 b of the comparator 40. The detection signal is conveyedfrom the second inductor 24 to the first inductor 16 via mutualinductance across the gaps 23, 29 (e.g., signal 25). The terminationresistance 28 provides termination impedance to the first inductor 16which is reflected back to the second inductor 18 via transformeraction. This reflected termination impedance develops a reflectedsignal, referred to as a reflected impedance signal. The reflectedimpedance signal is provided to the input terminal 40 a.. For example,if the first inductor 16 is not within communication range of the secondinductor 18, no reflected impedance signal is generated, and the valueof voltage provided to the input terminal 40 a is indicative of thisconfiguration. If the first inductor 16 is within communication range ofthe second inductor 18, a reflected impedance signal is created and thevalue of the voltage provided to input terminal 40 a is indicative ofthis configuration and differs from the value of the voltage provided tothe input terminal 40 a when the inductors 16, 18, are withincommunication range of each other. The comparator 40 provides acomparison signal 41, which is indicative of the status of thedetermination if the first portion 12 is within communication range ofthe second portion 14. If the signal at terminal 40 a is greater thanthe signal at terminal 40 b, the comparison signal will provide oneindication, and if the signal at terminal 40 a is less than the signalat terminal 40 b, than the comparison signal will provide anotherindication. Thus, the detection circuitry may be configured to indicatethat the first and second portions 12, 14, are within communicationrange of each other by selecting specific (predetermined) values ofresistance 34 and 36. Selecting predetermined values of resistances 34and 36 provides a predetermined portion of the detection signal beingprovided to the input terminal 40 b of the comparator 40. The values ofthe resistances 34 and 36 may be determined analytically, empirically,or a combination thereof. The comparison signal 41 is provided to theexclusive or gate 30, which also receives the detection signal. Anindicator signal 43 is provided by the exclusive or gate 30. Theindicator signal 43 is indicative of the determination if the firstportion 12 and the second portion 14 are within communication range ofeach other. Response to the indicator signal 43 may be by the operatorand/or may occur automatically. For example, upon receipt of theindicator signal 43, the radio operator (or the system) may choose toignore the indicator signal 43, terminate transmission, not commencetransmission, or a combination thereof.

[0027] As mentioned above, secure communications are facilitated if theattenuation of a wireless signal increases as the distance increasesfrom the source of radiation of the wireless signal. Thus, the morerapidly the attenuation increases, as a function of distance, the moresecure the system. Existing systems attenuate the radiated signal as afunction of the distance from the source of radiation, r, raised to thesecond power (1/r²). It has been shown, analytically, that a securewireless connection in accordance with the present invention providesattenuation of the radiated wireless signal increases as a function ofthe distance from the source of radiation raised to the third power(1/r³). A system providing attenuation as a function of 1/r³ is moresecure than a system providing attenuation as a function of 1/r².

[0028]FIG. 3 is a graphical plot of attenuation versus range forattenuation in accordance with a secure wireless connection inaccordance the present invention (curve 50), as a function of r⁴ (curve52), as a function of r³ (curve 54), and as a function of r² (curve 56).It can be shown that the attenuation of a radiated electromagneticsignal (e.g., signal 25) is attenuated as a function of range, F(r), inaccordance with the following equation. $\begin{matrix}{{{F(r)} = {\frac{1}{r^{2}} - \frac{1}{( {r + a} )^{2}}}},} & (1)\end{matrix}$

[0029] wherein:

[0030] F(r) is a function of r, r is the average distance from the twogaps (see FIG. 1), and “a” is a constant value equal to the distancebetween the two ends of an inductor (see FIG. 1).

[0031] Curve 50 is a plot of F(r), curve 52 is a plot of 1/r⁴, curve 54is a plot of 1/r³, and curve 56 is a plot of 1/r². As shown in FIG. 3,the curve 50 is closer to curve 54 than the other curves (50, 56). Thusthe function F(r) is closest to attenuation as a function of 1/r³. Also,only curves 52 and 54 appear to have the same shape. If curves 52 and 54have the same shape than the two curves are proportional.

[0032]FIG. 4 is a graph of the difference between curves 56 and 50(curve 58), curves 54 and 50 (curve 60), and curves 52 and 50 (curve62). Curve 58 is a plot of the difference between 10 log 1/r² (curve56)−10 log F(r) (curve 50). As shown in FIG. 4, curve 58 is not astraight line, thus indicating that curves 50 and 56 are not the sameshape. Curve 62 is a plot of the difference between 10 log 1/r⁴ (curve52)−10 log F(r) (curve 50). As shown in FIG. 4, curve 62 is not astraight line, thus indicating that curves 50 and 52 are not the sameshape. Curve 60 is a plot of the difference between 10 log 1/r³ (curve54)−10 log F(r) (curve 50). As shown in FIG. 4, curve 60 is a straightline, thus indicating that curves 50 and 54 are the same shape, and thusproportional. Therefore, a secure wireless connection in accordance withthe present invention provides attenuation of a radiated electromagneticsignal (e.g., signal 25) that is inversely proportional to the distancefrom the gaps raised to the third power (i.e., proportional to 1/r³).

[0033]FIG. 5 is a flow diagram of a process for providing wirelesssecure communication in accordance with an embodiment of the presentinvention. The first portion 12 and the second portion 14 are aligned atstep 70. Aligning the first and second portion 12, 14, includes aligningthe first end 20 of the first inductor 16 with the third end 24 of thesecond inductor 18, and aligning the second end 22 of the first inductor16 with the fourth end 26 of the second inductor 18. When aligned, theinductors, 16, 18, are configured to form a dual-gap toroidal core. Asdescribed above, the polarities of the electromagnetic field created ateach gap 23, 29, have opposite polarities. Thus, the opposite polarityfields cancel each other when combined (e.g., far field). Also, asdescribed above, the attenuation of radiated electromagnetic energy fromthe gaps 23, 29, is attenuated in accordance with an equationproportional to 1/r³.

[0034] At steps 72 through 82, it is determined if the first portion 12and the second portion 14 are within communications range of each other.At step 72, the detection signal (which may be amplifier and/orfiltered) is provided to the second inductor 18. At step 74, thedetection signal is conveyed from the second inductor 18 to the firstinductor 16 via wireless mutual inductance. An impedance signal iscreated at step 76 (e.g., via termination resistance 28). The impedancesignal is reflected back to the second inductor 18 via transformeraction (mutual inductance) generating a reflected impedance signal atstep 78. At step 80, a predetermined portion (e.g., via voltage dividercircuitry) of the detection signal is provided to the comparator (e.g.,comparator 40). The predetermined portion of the detection signal andthe reflected impedance signal are compared with each other at step 82.If the first and second portions 12, 14, are within communication range,as determined at step 78, the secure wireless communications via mutualinductance via the gaps, 23, 29, of the dual-gap toroidal core may beconducted. If it is determined, at step 78, that the first and secondportion 12, 14, are not within communication range of each other, an outof range indication is provided ate step 86. As described above, variouscourses of action may be pursued as a result of this out of rangeindication, such as manually and/or automatically ignoring the out ofrange indication, terminating communications, not initiatingcommunications, or a combination thereof.

[0035] A secure wireless connection in accordance with the presentinvention facilitates secure communications by attenuating a radiatedsignal proportional to 1/r³, where r is the average distance from thetwo gaps of the dual-gap toroidal core. This secure wireless connectionalso provides a communications link between two self contained portionsof a system through low frequency (e.g., voice or base band, low datarate signal) magnetic fields that can penetrate the envelope of theequipment when aligned with each other. Secure communications are alsofacilitated by the low power level of the transmitted signal, by eddycurrents formed in the walls of the first and second portions 12, 14,and by aligning the first and second portions 12, 14. The securewireless connection in accordance with the present invention does notrequire mechanical connectors, such as threaded connectors and/orbayonet connectors, thus reducing the weight and set up time. A systemcomprising the secure wireless connection in accordance with the presentinvention, such as a radio communication system, is capable of beingconfigured to allow water tight integrity, military hardening, and doesnot require modification of the structure (envelope) of the first andsecond portions, 12, 14, to facilitate communications. Because no changein material is required, such as occurs when a window is put into ametallic envelope to support an infrared link, the envelope of theequipment is not compromised.

[0036] Although illustrated and described herein with reference tocertain specific embodiments, the wireless secure connection asdescribed herein is nevertheless not intended to be limited to thedetails shown. Rather, various modifications may be made in the detailswithin the scope and range of equivalents of the claims and withoutdeparting from the spirit of the invention.

What is claimed is:
 1. A secure wireless communication systemcomprising: a first portion comprising a first inductor having a firstend and a second end; a second portion comprising a second inductorhaving a third end and a fourth end, wherein: said first portion andsaid second portion define a gap therebetween; and wirelesscommunication is achieved across said gap through mutual inductancebetween said first inductor and said second inductor by aligning saidfirst end with said third end, and said second end with said fourth end.2. A system in accordance with claim 1, wherein: said first inductor isone of rectilinear and arcuate shaped; and said second inductor is oneof rectilinear and arcuate shaped.
 3. A system in accordance with claim1, wherein upon alignment said first and second inductors are configuredto form a dual-gap toroidal core, electromagnetic energy radiating fromeach of said gaps having opposing polarities.
 4. A system in accordancewith claim 3, wherein attenuation of a combined electromagnetic fieldresulting from said gaps is inversely proportional to a distance fromsaid gaps raised to a third power.
 5. A system in accordance with claim1, further comprising detection circuitry for determining if said firstportion and said second portion are within communication range of eachother.
 6. A system in accordance with claim 5, said detection circuitrycomprising: a termination resistance electrically coupled to said firstinductor, said termination resistance configured to provide a reflectedimpedance signal from said first inductor to said second inductorresponsive to a detection signal; and a comparator electrically coupledto said second inductor and configured to receive said detection signal,wherein: said comparator compares said detection signal with saidreflected impedance signal to determine if said first portion and saidsecond portion are within communication range of each other.
 7. A systemin accordance with claim 6, further comprising a sense resistanceelectrically coupled to said second inductor for sensing said reflectedimpedance signal and providing indication of said reflected impedancesignal to said comparator.
 8. A system in accordance with claim 6,further comprising a voltage divider circuit for providing apredetermined portion of said detection signal to said comparator.
 9. Asystem in accordance with claim 1, wherein said first portion comprisesa military radio and said second portion comprises one of a faceplateand a headset for conveying information to said military radio.
 10. Asystem in accordance with claim 1, wherein information conveyed by saidwireless communication system comprises a cryptographic key.
 11. Amethod for providing secure wireless communications, said methodcomprising: aligning a first portion of communication system having afirst inductor with a second portion of a communication system having asecond inductor, wherein when aligned said first and second inductorsform a dual-gap toroidal core; conveying a wireless communication signalacross said gaps between said first and second portions via said firstand second inductors.
 12. A method in accordance with claim 11, furthercomprising: determining if said first portion is within communicationrange of said second portion.
 13. A method in accordance with claim 12,said step of determining comprising: providing a detection signal tosaid first inductor; receiving a reflected impedance signal from saidsecond inductor; and comparing said detection signal with said reflectedimpedance signal.
 14. A method in accordance with claim 12, furthercomprising: providing an out of range indication signal if said firstportion is determined not to be within communication range of saidsecond portion.
 15. A method in accordance with claim 11, whereinelectromagnetic energy radiating from said gaps of said dual-gaptoroidal core have opposing polarities.
 16. A method in accordance withclaim 11, wherein attenuation of a combined electromagnetic fieldresulting from gaps of said dual-gap toroidal core is proportional to adistance from said gaps raised to a third power.
 17. A method inaccordance with claim 11, wherein: said first inductor is one ofrectilinear and arcuate shaped; and said second inductor is one ofrectilinear and arcuate shaped.
 18. A method in accordance with claim11, wherein said wirelessly conveyed communication signal is indicativeof a cryptographic key.
 19. A wireless secure connection comprising: afirst inductor having a first end and a second end; a second inductorhaving a third end and a fourth end, wherein: said first end and saidthird end define a first gap therebetween; said second end and saidfourth end define a second gap therebetween; and said connector isconfigured to provide wireless communications across said gaps viamutual inductance between said first inductor and said second inductor.20. A connection in accordance with claim 19, wherein: said firstinductor is one of rectilinear and arcuate shaped; and said secondinductor is one of rectilinear and arcuate shaped.
 21. A connection inaccordance with claim 19, wherein: said first inductor and said secondinductor are configured to form a dual-gap toroidal core comprising saidfirst gap and said second gap; and electromagnetic energy radiating fromsaid first gap has an opposite polarity from electromagnetic energyradiating from said second gap.
 22. A connection in accordance withclaim 19, wherein: attenuation of a combined electromagnetic fieldresulting from electromagnetic energy radiating from said first gap andelectromagnetic energy radiating from said second gap is inverselyproportional to a distance from said gaps raised to a third power.